The study plans to enroll over 200 participants with SCID. By studying new participants undergoing treatment for SCID, the goal is to learn more about: (1) outcomes from the treatment of SCID in the modern era of medicine (2) what factors lead to the best long-term outcomes, such as best donor, conditioning regimen, timing of transplant, etc., and (3) what impact newborn screening and the early diagnosis of SCID has had on the long-term outcomes following BMT or gene therapy. Information is also being gathered on how and when the immune system recovers after BMT, quality of life for long-term survivors, and about whether children develop normally after treatment.